1. Field
The present application relates generally to wireless communications, and more specifically to methods and systems for efficient frequency assignment for mobile devices in coexisting wireless communication systems.
2. Background
Wireless communication systems are widely deployed to provide various types of communication (e.g., voice, data, multimedia services, etc.) to multiple users. As the demand for high-rate and multimedia data services rapidly grows, there lies a challenge to implement efficient and robust communication systems with enhanced performance.
In recent years, users have started to replace fixed line communications with mobile communications and have increasingly demanded great voice quality, reliable service, and low prices.
In addition to mobile phone networks currently in place, a new class of small base stations has emerged, which may be installed in a user's home and provide indoor wireless coverage to mobile units using existing broadband Internet connections. Such personal miniature base stations are generally known as localized communication systems, access point base stations, Home Node B's (HNB's), or femtocells. Typically, such miniature base stations are connected to the Internet and the mobile operator's network via DSL router or cable modem.
A prioritized list such as a Preferred Roaming List (PRL) is a database residing within many wireless devices, such as cellphones, that contains information used during the process of system selection and acquisition. In the case of RUIM-based CDMA devices, the PRL resides on the RUIM. The PRL indicates which bands, sub bands and service provider identifiers will be scanned and in what priority order. Without a PRL, the device may not be able to roam, i.e. obtain service outside of the home area. There may be cases where missing or corrupt PRL's can lead to a customer not having service at all.
On many networks, regularly updating the PRL is advised if the subscriber uses the device outside the home area frequently, particularly if they do so in multiple different areas. This allows the phone to choose the best roaming carriers, particularly “roaming partners” with whom the home carrier has a cost-saving roaming agreement, rather than using non-affiliated carriers. PRL files can also be used to identify home networks along with roaming partners, thus making the PRL an actual list that determines the total coverage of the subscriber, both home and roaming coverage.
The PRL is built by an operator and is normally not accessible to the user. Many operators provide the ability for the user to download the latest PRL to their device by dialing the Over-the-air (OTA) feature code *228.
A typical PRL structure consists of an acquisition table and a system table (along with some header and overhead information). The acquisition table comprises an indexed list of frequencies on which the device may search for particular systems. The idea behind the acquisition table is to optimize the acquisition time by identifying only the frequencies that should be searched, rather than searching the entire frequency spectrum. The information contained in each acquisition table entry includes an index, the network type, and associated channel blocks.
The system table generally comprises a prioritized list of systems that the device is permitted to access (Preferred Systems) and those that it is explicitly forbidden to access (Negative Systems). Note that it is also possible to indicate forbidden systems using a separate system identification number/network identification number (SID/NID) Lockout List in the handset; however, this is not a recommended approach as it creates control over system selection outside of the PRL.
Each system table entry belongs to a geographic area known as a GEO. These GEOs are listed in priority order. Each IS-95/1xRTT system is identified by either SID/NID or, in the case of enhanced PRL, SID/NID or MCC/MNC. To support 1xEV-DO systems, the PRL must be IS-683-C or later. Each 1xEV-DO system is identified by a Subnet ID. For hybrid mode operation (i.e. IS-95/1xRTT and 1xEV-DO), association tags are used link an IS-95/1xRTT system with one or more 1xEV-DO systems. Hybrid systems always attempt to select an IS-95/1xRTT system first, then use the association tag to attempt to select an associated 1xEV-DO system.
Each entry also provides an acquisition table index where the frequencies associated with that system are identified and a roaming indicator that dictates what type of indication should be displayed to the user when they are receiving service from that system.
The PRL typically operates in one of two modes, a restrictive mode or a permissive mode, dictated by a Preferred Only indicator. In the restrictive mode, otherwise known as the closed PRL or Preferred Only Mode, the device will only acquire systems identified as preferred in the PRL system table. While this mode of operation gives operators the most control over the system selection process, it requires that PRLs be kept up-to-date to ensure that newly added roaming partners can be selected. Note that even if a PRL is restrictive according to the preferred only indicator, the use of wildcard SID/NID or MCC/MNC values as preferred systems would essentially make the PRL permissive because wildcards match any system that is located.
In the permissive mode, otherwise known as the open PRL or Not Preferred Mode, the device will attempt to acquire preferred systems. However, if no preferred systems can be located, the device may select systems that are not listed in the PRL system table. It still will not select negative systems; but, if no preferred systems can be found and an unknown system is available, it will select the unknown system. While this mode of operation is easier to maintain because new partners can be chosen even if they are not explicitly listed in the PRL, it carries with it the danger of allowing the mobile device to become camped onto a system that will not provide it with service. Further information on PRL is defined by OTASP specification TIA-683, available from the 3GPP2 website as C.S0016.
Prior mechanisms used for a UE to access femtocells include using beacons deployed on all macro carriers (i.e., carrier signals emanating from cellular communication systems), a PRL mechanism within the UE that lists the femtocell as the most preferred system with all the macro channels treated as equal, or using the macro system to send one or more SRDMs (redirection messages) to move the mobile to a femto-channel associated with a femtocell.
One shortcoming of the aforementioned methods to access femtocells is that a significant amount of power may be consumed by a UE attempting to locate and connect with a femtocell. Therefore, there is a need to enable UEs to locate and connect to femtocells while minimizing power requirements to do so.